Blacksmith in plains village have a strange room

The bug

Blacksmith in plains village have a strange room. I don't kown what is it but monsters spawn inside it. I don't know if it WAI bug normally no.

This happens since 18w48a.

How to reproduce

• Create a new superflat world with this seed: 

  -2608543111006229119

• And teleport to

  /tp @s 125.05 9.00 776.37 -1893.74 13.05